The present invention concerns digital television receivers and in particular a receiver including multiple program decoders and/or multiple tuners to allow programs from successive channels to be quickly displayed and selected.
Digital television programming is available from many sources, these include satellite broadcasts, digital cable systems and terrestrial broadcasts systems. A viewer connected to a digital cable system may, for example, be able to receive up to 500 channels. This is possible because the digital television signals are compressed to eliminate redundancies from line to line and frame to frame and, thus, the required bandwidth needed to transmit the signals is also reduced.
The compression technology used to achieve this efficiency, however, also makes the signals difficult to decode and display. A typical motion compensated predicted frame, for example, is encoded in many steps. First corresponding image data from a previous frame, as determined by a motion compensation processor, is subtracted from the image data in the current frame. Next, this difference data is encoded using a discrete cosine transform process. The discrete cosine transform coefficients are applied to a variable quantiser to reduce the number of bits used to represent each coefficient. Next, the quantized coefficients are processed through a variable length coder and run length coder to further reduce the number of bits needed to represent the video signal. The resulting bit-stream is packetized into variable-length program elementary stream (PES) packets which are further decomposed into fixed-length transport packets. The packetized bit-stream is further annotated with information needed to recover the bit-stream from the packets. Finally, forward error correction code information is added to the signal. The encoded signal is then modulated onto a radio frequency (RF) virtual side band carrier signal for terrestrial broadcast video systems or on to a quadrature amplitude modulated carrier for cable broadcast systems.
The decoding operations to reproduce the video signal must reverse the operation performed by the encoder in order to produce a decoded signal. A typical digital television receiver locks a tuner on to the pilot signal of the RF digital television signal then it demodulates the RF television signal to produce a baseband encoded television signal. Next, the forward error correction coding is processed to recover the transport packets. The transport packets are then processed to reproduce the PES packets and the PES packets are processed to reproduce the elementary bit-stream. The elementary bit-stream for the video signal is applied to a variable length decoder and run-length decoder which produce fixed length code values representing the frequency domain coefficients as well as other side information (e.g. motion vectors) needed to decode the video signal. Next, the fixed length coefficient values are applied to an inverse quantiser and to an inverse discrete cosine transform processor to reproduce pixel values.
If the frame being decoded is a motion compensated frame, the pixel values are differential pixel values that are referenced to a pixel values in a prior frame or field as indicated by a motion vector. This motion vector is received as part of the side information in the transmitted signal. The pixels represented by the motion vector are recovered and added to the differential pixels in order to reproduce the video signal.
Similar digital encoding techniques are used for the audio portion of the television signal.
The complex coding scheme used for digital television programming produces great coding efficiency but causes delay in the reproduction of a video signal. The time between when a television tuner is first tuned to a digital television channel and the time that the video information is displayed may be as long as 1 to 4 seconds.
Channel surfing is a process by which a viewer successively tunes a television receiver to one channel after another in order to find a program that he or she wishes to watch. On conventional analog television receivers, the time between the display of images for successive channels in a channel surfing operation is less than one second. The increase in the amount of time needed to display images from successive digital television programs may frustrate even the most patient channel surfer.
In order to over come the problem of channel surfing many cable systems provide electronic program guides (EPG). These guides allow a viewer to bring up a description of programming currently available on multiple channels, typically in a grid form. By visually scanning the grid a viewer may determine the programming on any of a number of channels and tune to a channel by selecting an item from the grid. It is difficult, however, to succinctly describe a television program in a grid format. Consequently many viewers perceive these guides as boring or of limited utility.
Another system which has been proposed is described in U.S. Pat. No. 5,532,748 entitled Hybrid Analog/Digital Television Transmission System. This patent describes a system that includes an analog video signal along with the digital television signal to produce a hybrid television signal for use during channel change operations. When a viewer using this system changes channels the low-resolution analog signal is displayed at first followed by the digital signal when the digital decoder has enough information. This system, however, does not allow digital systems to retain the high quality picture during channel surfing which users have come to expect from digital systems.
The present invention is embodied in a digital television receiver that decodes multiple programs concurrently to reduce the amount of time needed to display an adjacent channel in a channel surfing operation.
One embodiment of the receiver employs a single transport decoder and a single video/audio decoder. The transport decoder concurrently decodes multiple transport streams in a single channel. Each transport stream represents a respectively different minor channel transmitted in the major channel. When a channel change operation occurs, the audio/video decoder selects the next transport stream that is provided by the transport decoder.
According to a second embodiment of the invention, the single video/audio decoder includes multiple buffers and concurrently decodes multiple program streams. When a program switch occurs, the program stream for a next buffer is provided to the video display.
According to another embodiment of the invention, the television receiver includes multiple tuners, each capable of receiving a respective channel. Each tuner is coupled to a transport decoder and each transport decoder is coupled to a respective video/audio decoder. In this embodiment of the invention, when a channel switch occurs, the video signal is immediately available at the output of one of the decoders and is displayed. At the same time, one of the tuners is tuned to the next channel in the sequence in anticipation of a subsequent channel switch operation.